The first version of --harmony-for-in avoided throwing PreParser
errors in order to retain use counting. This patch threads
use_counts_ through to the PreParser to allow use counting in
the PreParser while also throwing errors for this case.

Also slightly refactored the Parser code to do a little less
code duplication.

BUG=v8:4942
LOG=y

Review URL: https://codereview.chromium.org/1913203002

Cr-Commit-Position: refs/heads/master@{#35780}